diff --git a/cmake/preload/toolchains/pico_arm_cortex_m0plus_clang.cmake b/cmake/preload/toolchains/pico_arm_cortex_m0plus_clang.cmake
index 5ebde3e00..d445d3c73 100644
--- a/cmake/preload/toolchains/pico_arm_cortex_m0plus_clang.cmake
+++ b/cmake/preload/toolchains/pico_arm_cortex_m0plus_clang.cmake
@@ -1,8 +1,5 @@
 set(CMAKE_SYSTEM_PROCESSOR cortex-m0plus)
 
-# these are all the directories under LLVM embedded toolchain for ARM (newlib or pibolibc) and under llvm_libc
-set(PICO_CLANG_RUNTIMES armv6m_soft_nofp armv6m-unknown-none-eabi)
-
-set(PICO_COMMON_LANG_FLAGS "--target=armv6m-none-eabi -mfloat-abi=soft -march=armv6m")
+set(PICO_COMMON_LANG_FLAGS "--target=arm-none-eabi -mcpu=cortex-m0plus -mfloat-abi=soft")
 
 include(${CMAKE_CURRENT_LIST_DIR}/util/pico_arm_clang_common.cmake)
diff --git a/cmake/preload/toolchains/pico_arm_cortex_m33_clang.cmake b/cmake/preload/toolchains/pico_arm_cortex_m33_clang.cmake
index da42da46e..16f30327e 100644
--- a/cmake/preload/toolchains/pico_arm_cortex_m33_clang.cmake
+++ b/cmake/preload/toolchains/pico_arm_cortex_m33_clang.cmake
@@ -1,8 +1,6 @@
 set(CMAKE_SYSTEM_PROCESSOR cortex-m33)
 
-# these are all the directories under LLVM embedded toolchain for ARM (newlib or pibolibc) and under llvm_libc
-set(PICO_CLANG_RUNTIMES armv8m.main_soft_nofp armv8m.main_soft_nofp_unaligned armv8m.main-unknown-none-eabi)
-
-set(PICO_COMMON_LANG_FLAGS "-mcpu=cortex-m33 --target=armv8m.main-none-eabi -mfloat-abi=softfp -march=armv8m.main+fp+dsp")
+set(PICO_COMMON_LANG_FLAGS "--target=arm-none-eabi -mfloat-abi=hard -march=armv8m.main+fp+dsp")
 set(PICO_DISASM_OBJDUMP_ARGS --mcpu=cortex-m33 --arch=armv8m.main+fp+dsp)
+
 include(${CMAKE_CURRENT_LIST_DIR}/util/pico_arm_clang_common.cmake)
diff --git a/cmake/preload/toolchains/util/pico_arm_clang_common.cmake b/cmake/preload/toolchains/util/pico_arm_clang_common.cmake
index 5cb4e2df5..bd9ad0df9 100644
--- a/cmake/preload/toolchains/util/pico_arm_clang_common.cmake
+++ b/cmake/preload/toolchains/util/pico_arm_clang_common.cmake
@@ -11,7 +11,7 @@ pico_find_compiler(PICO_OBJCOPY llvm-objcopy)
 pico_find_compiler(PICO_OBJDUMP llvm-objdump)
 
 # Specify the cross compiler.
-set(CMAKE_C_COMPILER ${PICO_COMPILER_CC} CACHE FILEPATH "C compiler")
+set(CMAKE_C_COMPILER   ${PICO_COMPILER_CC} CACHE FILEPATH "C compiler")
 set(CMAKE_CXX_COMPILER ${PICO_COMPILER_CXX} CACHE FILEPATH "C++ compiler")
 set(CMAKE_ASM_COMPILER ${PICO_COMPILER_ASM} CACHE FILEPATH "ASM compiler")
 
@@ -43,55 +43,49 @@ endforeach()
 
 list(APPEND CMAKE_TRY_COMPILE_PLATFORM_VARIABLES PICO_CLIB)
 
-foreach(PICO_CLANG_RUNTIME IN LISTS PICO_CLANG_RUNTIMES)
-    # LLVM embedded-toolchain for ARM style
-    find_path(PICO_COMPILER_SYSROOT NAMES include/stdio.h
-            HINTS
-            ${PICO_COMPILER_DIR}/../lib/clang-runtimes/arm-none-eabi/${PICO_CLANG_RUNTIME}
-            ${PICO_COMPILER_DIR}/../lib/clang-runtimes/${PICO_CLANG_RUNTIME}
-    )
-
-    if (PICO_COMPILER_SYSROOT)
-        if (NOT PICO_CLIB)
-            # this is a bit of a hack; to try to autodetect the C library used:
-            # `picolibc.h` seems to exist on the newer versions of LLVM embedded toolchain for ARM using picolibc whereas
-            # `newlib.h` appears in all versions, so isn't very useful
-            if (EXISTS "${PICO_COMPILER_SYSROOT}/include/picolibc.h")
-                message("Setting default C library to picolibc as LLVM appears to be using it")
-                set(PICO_CLIB "picolibc" CACHE INTERNAL "")
-            endif()
-        endif()
-        break()
+set(_CLANG_RUNTIMES_DIR "${PICO_COMPILER_DIR}/../lib/clang-runtimes")
+cmake_path(NORMAL_PATH _CLANG_RUNTIMES_DIR)
+set(PICO_CLIB_ROOT "${_CLANG_RUNTIMES_DIR}")
+
+if(NOT PICO_CLIB  OR  PICO_CLIB STREQUAL "")
+    # newlib is primary if no clib specified 
+    if(EXISTS "${_CLANG_RUNTIMES_DIR}/newlib")
+        set(PICO_CLIB "newlib")
+    else()
+        set(PICO_CLIB "picolibc")
     endif()
-    # llvm_libc style
-    find_path(PICO_COMPILER_SYSROOT NAMES stdio.h
-            HINTS
-            ${PICO_COMPILER_DIR}/../include/${PICO_CLANG_RUNTIME}
-    )
-    if (PICO_COMPILER_SYSROOT)
-        if (NOT PICO_CLIB)
-            message("Setting default C library to llvm_libc as LLVM appears to be using it")
-            set(PICO_CLIB "llvm_libc" CACHE INTERNAL "")
-        endif()
-        break()
+    set(CACHE{PICO_CLIB} TYPE STRING FORCE VALUE ${PICO_CLIB})
+endif()
+
+if(PICO_CLIB STREQUAL "newlib")
+    if(EXISTS "${_CLANG_RUNTIMES_DIR}/newlib")
+        set(PICO_CLIB_ROOT "${_CLANG_RUNTIMES_DIR}/newlib")
     endif()
-endforeach()
+elseif(PICO_CLIB STREQUAL "llvm_libc")
+    if(EXISTS "${_CLANG_RUNTIMES_DIR}/llvmlibc")
+        set(PICO_CLIB_ROOT "${_CLANG_RUNTIMES_DIR}/llvmlibc")
+    endif()
+elseif(PICO_CLIB STREQUAL "picolibc")
+    if(EXISTS "${_CLANG_RUNTIMES_DIR}/picolibc")
+        set(PICO_CLIB_ROOT "${_CLANG_RUNTIMES_DIR}/picolibc")
+    endif()
+else()
+    message(FATAL_ERROR "PICO_CLIB must be one of newlib, picolib, llvm_libc or empty (but is '${PICO_CLIB}')")
+endif()
+
+set(PICO_COMMON_LANG_FLAGS "${PICO_COMMON_LANG_FLAGS} --sysroot ${PICO_CLIB_ROOT}")
 
 # moving this here as a reminder from pico_standard_link; it was commented out theee, but if ever needed,
 # it belongs here as part of LINKER_FLAGS_INIT
 #target_link_options(pico_standard_link INTERFACE "LINKER:-fuse-ld=lld")
 
 if (PICO_CLIB STREQUAL "llvm_libc")
-    # TODO: Remove -nostdlib++ once we include libc++ in the toolchain.
     # TODO: Move -nostartfiles to the appropriate library.
     foreach(TYPE IN ITEMS EXE SHARED MODULE)
-        set(CMAKE_${TYPE}_LINKER_FLAGS_INIT "-nostdlib++ -nostartfiles")
+        set(CMAKE_${TYPE}_LINKER_FLAGS_INIT "-nostartfiles")
     endforeach()
-else()
-    if (NOT PICO_COMPILER_SYSROOT)
-        message(FATAL_ERROR "Could not find an llvm runtime for '${PICO_CLANG_RUNTIME}'")
-    endif()
-
-    set(PICO_COMMON_LANG_FLAGS "${PICO_COMMON_LANG_FLAGS} --sysroot ${PICO_COMPILER_SYSROOT}")
 endif()
+
+message(STATUS "Taking '${PICO_CLIB}' from '${PICO_CLIB_ROOT}'")
+
 include(${CMAKE_CURRENT_LIST_DIR}/set_flags.cmake)
diff --git a/src/rp2_common/pico_clib_interface/BUILD.bazel b/src/rp2_common/pico_clib_interface/BUILD.bazel
index 9bb05a99c..8ddc02de2 100644
--- a/src/rp2_common/pico_clib_interface/BUILD.bazel
+++ b/src/rp2_common/pico_clib_interface/BUILD.bazel
@@ -98,10 +98,16 @@ cc_library(
     target_compatible_with = compatible_with_rp2(),
 )
 
-# For now, picolibc doesn't need to provide any headers.
-alias(
+cc_library(
     name = "picolibc_interface",
-    actual = "//bazel:empty_cc_lib",
+    hdrs = [
+        "include/picolibc/sys/cdefs.h",
+    ],
+    includes = ["include/picolibc"],
+    # It's hard to properly constrain compatibility since `auto` may select this,
+    # so just tag as manual.
+    tags = ["manual"],
+    target_compatible_with = compatible_with_rp2(),
 )
 
 cc_library(
diff --git a/src/rp2_common/pico_clib_interface/CMakeLists.txt b/src/rp2_common/pico_clib_interface/CMakeLists.txt
index 84d3faf62..e080bd34d 100644
--- a/src/rp2_common/pico_clib_interface/CMakeLists.txt
+++ b/src/rp2_common/pico_clib_interface/CMakeLists.txt
@@ -21,6 +21,10 @@ if (NOT TARGET pico_clib_interface)
     #            PICO_STDIO_SHORT_CIRCUIT_CLIB_FUNCS=0
     #)
 
+    target_include_directories(pico_picolibc_interface SYSTEM INTERFACE
+            ${CMAKE_CURRENT_LIST_DIR}/include/picolibc
+    )
+    
     #  ---- llvm_libc ----
     pico_add_library(pico_llvm_libc_interface)
 
diff --git a/src/rp2_common/pico_clib_interface/include/llvm_libc/time.h b/src/rp2_common/pico_clib_interface/include/llvm_libc/time.h
index bb37c7eff..df3d11d29 100644
--- a/src/rp2_common/pico_clib_interface/include/llvm_libc/time.h
+++ b/src/rp2_common/pico_clib_interface/include/llvm_libc/time.h
@@ -9,15 +9,15 @@
 
 #include <__llvm-libc-common.h>
 
+#include <llvm-libc-types/struct_timespec.h>
 #include <llvm-libc-types/struct_tm.h>
 #include <llvm-libc-types/time_t.h>
 
 __BEGIN_C_DECLS
 
 struct tm* localtime_r(const time_t* timer, struct tm* buf);
+time_t mktime(struct tm *);
 
 __END_C_DECLS
 
-#include_next <time.h>
-
 #endif // _PICO_LLVM_LIBC_TIME_H
diff --git a/src/rp2_common/pico_clib_interface/include/picolibc/sys/cdefs.h b/src/rp2_common/pico_clib_interface/include/picolibc/sys/cdefs.h
new file mode 100644
index 000000000..5492e7864
--- /dev/null
+++ b/src/rp2_common/pico_clib_interface/include/picolibc/sys/cdefs.h
@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef __PICO_PICOLIBC_SYS_CDEFS_H
+#define __PICO_PICOLIBC_SYS_CDEFS_H
+
+#if defined(__STDC__) || defined(__cplusplus)
+
+#define __CONCAT1(x,y)  x ## y
+#define __CONCAT(x,y)   __CONCAT1(x,y)
+#define __STRING(x)     #x
+#define __XSTRING(x)    __STRING(x)
+
+#endif
+
+#define __unused        __attribute__((__unused__))
+#define __used          __attribute__((__used__))
+#define __packed        __attribute__((__packed__))
+#define __aligned(x)    __attribute__((__aligned__(x)))
+
+#define __always_inline __inline__ __attribute__((__always_inline__))
+#define __noinline      __attribute__((__noinline__))
+
+#define __printflike(fmtarg, firstvararg) \
+  __attribute__((__format__ (__printf__, fmtarg, firstvararg)))
+
+#include_next <sys/cdefs.h>
+
+#endif
diff --git a/src/rp2_common/pico_clib_interface/llvm_libc_interface.c b/src/rp2_common/pico_clib_interface/llvm_libc_interface.c
index dfcab015e..c852bcb2e 100644
--- a/src/rp2_common/pico_clib_interface/llvm_libc_interface.c
+++ b/src/rp2_common/pico_clib_interface/llvm_libc_interface.c
@@ -7,11 +7,11 @@
 #include <math.h>
 #include <stdbool.h>
 #include <stddef.h>
-#include <sys/time.h>
-#include <time.h>
-
 #include <llvm-libc-types/ssize_t.h>
 
+#include "include/llvm_libc/time.h"
+#include "include/llvm_libc/sys/time.h"
+
 #include "pico/runtime_init.h"
 #include "pico/stdio.h"
 #include "pico/time.h"
diff --git a/src/rp2_common/pico_clib_interface/picolibc_interface.c b/src/rp2_common/pico_clib_interface/picolibc_interface.c
index 63d79135e..d73ef3f97 100644
--- a/src/rp2_common/pico_clib_interface/picolibc_interface.c
+++ b/src/rp2_common/pico_clib_interface/picolibc_interface.c
@@ -124,9 +124,19 @@ void runtime_init(void) {
     runtime_init_per_core_install_stack_guard(&__StackBottom);
 #endif
 
-    // piolibc __libc_init_array does __preint_array and __init_array
-    extern void __libc_init_array(void);
-    __libc_init_array();
+    // todo maybe we want to do this in the future, but it does stuff like register_tm_clones
+    //      which we didn't do in previous SDKs
+    //extern void __libc_init_array(void);
+    //__libc_init_array();
+
+    // ... so instead just do the __preinit_array
+    runtime_run_initializers();
+    // ... and the __init_array
+    extern void (*__init_array_start)(void);
+    extern void (*__init_array_end)(void);
+    for (void (**p)(void) = &__init_array_start; p < &__init_array_end; ++p) {
+        (*p)();
+    }
 }
 
 #if !PICO_RUNTIME_NO_INIT_PER_CORE_TLS_SETUP
@@ -151,4 +161,4 @@ PICO_RUNTIME_INIT_FUNC_PER_CORE(runtime_init_pre_core_tls_setup, PICO_RUNTIME_IN
 //            "ldr r0, =__tls_base\n"
 //            "bx lr\n"
 //            );
-//}
\ No newline at end of file
+//}